Page 255 - Refining Biomass Residues for Sustainable Energy and Bioproducts

Sweet sorghum: a potential resource for bioenergy production 225

yielded maximum total sugar of 909.4 g/kg dry bagasse than dilute NaOH treatment
with autoclaving concentrated NaOH, alkaline peroxide, and autoclaving
treatments.
In acid pretreatment, H 2 SO 4 , HNO 3 , HCl, and H 3 PO 4 have been used to hydro-
lyze the hemicelluloses. However, generation of hazardous compounds during the
pretreatment and carbohydrate loss are the major disadvantages (Rabemanolontsoa
and Saka, 2016). Deshavath et al. (2017) used dilute acid pretreatment (0.2 M

H 2 SO 4 ) on sweet sorghum stalks for 2 h at 121 C and achieved 89% xylan conver-
sion. On comparing three dilute-acid pretreatments, that is, HCl, H 2 SO 4 , and HCl/
H 2 SO 4 , which produced similar sugar yields, HCl pretreatment requires almost half
of the hydrolysis time than others (Heredia-Olea et al., 2012). Green liquor pretreat-
ment (Na 2 CO 3 and Na 2 S) on sweet sorghum bagasse reduced the lignin content and
helped in retaining the holocellulose content of the biomass at optimum condition
of 160 C for 110 min yielded maximum sugar of 82.6% (Pham et al., 2018).

Solvolysis or organic solvent-based pretreatments are efficient in removing lig-
nin and hemicellulose content, but the solvent cost is high, and the recovery is quite
challenging. Acetone, ethylene, ethanol, and methanol are generally applied in the
solvolysis pretreatment (Hassan et al., 2018). Acetone pretreated 1 kg sorghum
bagasse resulted in higher saccharification efficiency during enzymatic hydrolysis
and produced 78 g butanol, 35 g acetone, 12 g ethanol, 28 g acetic acid, and 6 g
butyric acid from 1 kg of sorghum bagasse (Jafari et al., 2016). Ostovareh et al.
(2015) used organosolv pretreatment—a combination of 50% ethanol and 1%
H 2 SO 4 at 140 C—on sweet sorghum stalks and obtained a maximum sugar yield

(77% of the theoretical yield).
Ionic liquids which have gained increased attention recently are the third generation
of solvents. Lignin can be efficiently removed by the ionic liquids. However, the ionic
liquids are difficult to recover after pretreatment. Most of the frequently applied ionic
liquids are 1-butyl-3-methylimidazolium hexafluorophosphate, 1-butyl-3-
methylimidazoliumacetate, 1-butyl-3-methylimidazoliumchloride, and N,N-dimethy-
lethanol-ammonium groups (Kumari and Singh, 2018). Sorghum bagasse was pre-
treated using 1-butyl-3-methylimidazolium chloride, and this resulted in the conversion
of 40% cellulose and produced 14 g/L of glucose after 60 h (Zhang et al., 2011).

10.3.3 Biological

Biological pretreatment is an eco-friendly route to separate the lignocellulosic bio-
mass. In biological pretreatment, fungi are the most suitable and efficient candi-
dates, which degrade the lignin, cellulose, and hemicellulose structure efficiently.
Some bacterial system, microbial consortium, and crude enzymes (laccases, Mn
peroxidase, lignin peroxidases, etc.) are also used to destruct the components.
Despite having advantages, this approach is slow and less efficient as compared to
the physical and chemical pretreatments. Fungi, a group from Basidiomycetes called
white-rot fungi, is capable of mineralizing the whole lignin structure along with
degradation of cellulose and hemicellulose (Rudakiya and Gupte, 2017). Numerous